The present invention generally relates to an apparatus for regulating the transmission of data within a data communication system and, in particular, relates to such an apparatus for the masterless control of data traffic among a plurality of closely spaced interface devices each having one or more external peripherals interconnected therewith.
In modern communication networks a plurality of data transmit/receive elements, each capable of transmitting and receiving data signals, are interconnected for the exchange of information therebetween. The interconnection is frequently by means of a common communication medium, such as a coaxial cable. Naturally, if more than one such element were to simultaneously transmit data onto the common communication medium excessive errors and data loss would occur and the network would be quite unacceptable. Consequently, one major consideration in the design of such communication networks must be the regulation, or allocation, of the transmission time among the subscribers, or users, in a fair and efficient manner so that undue delays or other inconveniences are avoided when attempting to transmit information. This consideration persists regardless of the magnitude of the network designed, i.e., a long-haul network, a local area network, or a short-haul local area network.
At the present time and, as anticipated for years to come, one of the primary systems of interest for many groups of users is either a local area network (LAN) or a short-haul local area network (SHLAN). Such networks are generally employed in geographically concentrated facilities, such as a university or a business site, and include a plurality of nodes each having a number of peripheral devices connected thereto.
At the present time, the control of such nodes is available on very large-scale integrated (VLSI) circuit chips. These nodal VLSI circuit chips are then interconnected to provide communication services with other similar nodes to form, for example, a short-haul local area network. This design consideration can be characterized as the avoidance of collisions between two or more users attempting to simultaneously transmit information over a common transmission medium that interconnects a plurality of nodes.
The most conventional solution to this congestion, or collision, problem is the use of a carrier signal. That is, an analog carrier signal is transmitted, usually via coaxial cable, to each and every node on the network. Each node is adapted to detect the presence of the carrier signal. The most common scheme is known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In such a scheme, the phrase "carrier sense" means that any data transceiver client, or subscriber, wishing to transmit "listens" first. If the communication medium is being used by another subscriber the client defers transmission. "Multiple access" indicates that any client wishing to transmit can do so without the need for a central controller. "Collision detection" defers to the fact that when the communication bus is idle any element can begin transmitting.
The carrier signal is detected at each node and serves to indicate to those nodes attempting to transmit that to do so would create a transmission collision. Consequently, when a carrier signal is detected, any other node attempting to transmit is restrained from transmitting. This restraint is often implemented by requiring an enabling signal to be provided before transmission occurs. Upon failure to receive such an enabling signal, for example, due to the detection of a carrier signal, transmission from the node is restrained.
The carrier signal, even for local area networks such as a large building, or a cluster of buildings, is, as mentioned, most frequently transmitted to each and every node via a coaxial cable. As a result, this solution can be quite expensive because the coaxial cable must be routed to each node of the network and requires expensive interface equipment.
From the above, it is apparent that schemes such as CSMA/CD for avoiding simultaneous transmission collisions become expensive due to the requirement of coaxial cable lengths and hard wire interconnections necessary to service every node.